Intraseasonal oscillation (ISO) of winter rainfall in southern China and its relationship with the circulation patterns in different latitudes are investigated using ERA‐Interim reanalysis data and observations. The results show that the rainfall in southern China has obvious ISO characteristics, with a dominant period of 10–30 days. The convergence of the whole layer integral water vapour flux can satisfactorily represent the rainfall rate, and the intraseasonal (10–30‐day filtered) rainfall is converted into a function of different time scale wind and water vapour. At the intraseasonal time scale, rainfall (10–30‐day filtered) is determined by both the 10–30‐day wind and the background (greater than 30 days) moisture. Nine days before the peak day of the intraseasonal rainfall in southern China, there is an abnormal low‐pressure system (10–30‐day filtered) in the polar region (east to Novaya Zemlya), and the low‐pressure system gradually strengthens because of the intraseasonal relative vorticity advection at high levels. The intraseasonal relative vorticity advection guides the intraseasonal geopotential anomaly to move from the high latitude to the low latitude, and the low‐pressure system at the surface also moves southward. There is no intraseasonal precipitation until the low‐pressure system at the surface moves to the region near southern China, because there is little water vapour in northern China and ample water vapour in the south. After the occurrence of precipitation, the vorticity advection from the high latitude no longer affects the strength of the low‐pressure system, but the intraseasonal diabatic heating accompanied by precipitation causes a second drop in pressure from −3 days to −1 day and makes the low‐pressure system strongest here. In addition, the geopotential anomaly (10–30‐day filtered) in low latitudes moves eastward along the southern foothills of the Tibetan plateau, which can guide the low‐pressure system to move southward until it reaches southern China.

Subseasonal-to-seasonal (S2S) prediction is a highly regarded skill around the world. To improve the S2S forecast skill, an S2S prediction project and an extensive database have been established. In this study, the European Center for Medium-Range Weather Forecasts (ECMWF) model hindcast, which participates in the S2S prediction project, is systematically assessed by focusing on the hindcast quality for the summer accumulated ten-day precipitation at lead times of 030 days during 19952014 in eastern China. Additionally, the hindcast error is corrected by utilizing the preceding sea surface temperature (SST). The metrics employed to measure the ECMWF hindcast performance indicate that the ECMWF model performance drops as the lead time increases and exhibits strong interannual differences among the five subregions of eastern China. In addition, the precipitation forecast skill of the ECMWF hindcast is best at approximately 15 days in some areas of Southeast China; after correcting the forecast error, the forecast skill is increased to 30 days. At lead times of 030 days, regardless of whether the forecast error is corrected, the root mean square errors are lowest in Northeast China. After correcting the forecast error, the performance of the ECMWF hindcast shows better improvement in depicting the quantity and temporal and spatial variation of precipitation at lead times of 030 days in eastern China. The false alarm ratio (FAR), probability of detection (POD), and equitable threat score (ETS) reveal that the ECMWF model has a preferable performance at forecasting accumulated ten-day precipitation rates of approximately 2050mm and indicates an improved hindcast quality after the forecast error correction. In short, adopting the preceding SST to correct the summer subseasonal precipitation of the ECMWF hindcast is preferable.

In August-September 2021, the western Pacific subtropical high (WPSH) showed significant anomalies, with some areas between the Yangtze and Yellow River basins in China suffering from heavy rainfall and flooding, while extreme heat occurred in Southern China. Using the 6 h reanalysis data of NCEP/NCAR and the daily precipitation observation data, this paper studied the abnormal characteristics and the thermal mechanism of the three-dimensional ridge of the WPSH during the seasonal transition from late summer to early autumn in August-September 2021, and explored its connection with the precipitation in the areas between the Yangtze and Yellow River basins in China during the same period. The results showed that, in August-September 2021, the three-dimensional ridge of the WPSH appeared to be southward at the lower troposphere and tilted northward with height. According to the diagnosis of the atmospheric heat source, it was found that the center of atmospheric cold source in the low latitudes at 850 hPa enhanced the surface high pressure, corresponding to the WPSH ridge shifting southward at the lower troposphere, which led to more water vapor convergence and more precipitation between the Yangtze and Yellow River basins. The latent heat of condensation released by the increased precipitation between the Yangtze and Yellow River basins in China heated the middle and upper troposphere, increasing the intensity of the high pressure in the upper troposphere, corresponding to the WPSH ridge northward at the upper troposphere. This anomalous tilt of the WPSH ridge in August-September 2021 and the intense precipitation processes between the Yangtze and Yellow River basins in China during the same period established a reciprocal feedback mechanism through diabatic heating. Further studies also showed that the anomalous tilt of the WPSH ridge is one of the important modes of its three-dimensional structural anomalies, and that, in years when the ridge tilts from south to north with height anomalously in August-September, the areas between the Yangtze and Yellow River basins are characterized by high precipitation.

Persistent heavy rainfall events (PHREs), as a result of the interaction among multiscale systems, are prone to continuously affect the middle and lower reaches of the Yangtze River valley (MLYR). Based on the observation and reanalysis data, a total of 41 PHREs during the mei-yu seasons of 1979–2020 are first identified over the MLYR and divided into prolonged (over 5 days) and normal (3–5 days) groups. The contributions of quasi-biweekly-scale and synoptic-scale components to the abovementioned two types of PHREs are analyzed. Prolonged PHREs are dominated by the quasi-biweekly component of precipitation (QBW_Pr), while normal PHREs depend on synoptic-scale components (SS_Pr). The quantitative moisture budget indicates that the favorable environment for the QBW_Pr of prolonged PHREs is associated with the interaction between background moisture and quasi-biweekly circulation disturbances. Approximately 80% of prolonged PHREs occurred in phases 6–8 of the boreal summer QBWO (BSISO2) life cycle. The large-scale meridional vertical circulation along the South China Sea (SCS)–MLYR, and the westward-propagating suppressed convection at low latitudes, accompanied by the substantial northward and upward moisture supply, significantly promotes and maintains the QBW_Pr for prolonged PHREs. In contrast, the QBWO signals are relatively insignificant in the evolution of normal PHREs. The impact of the low-latitude QBWO on the prolonged PHREs is further confirmed by sensitivity experiments with RegCM4.9. As the QBWO are weakened on the southeast lateral boundary near the SCS, the vertical circulation and moisture transport clearly diminish along the SCS–MLYR. Consequentially, precipitation reduces visibly during prolonged PHREs. Significance Statement: Persistent heavy rainfall events (PHREs), as the reflection of the interaction among multiscale circulation systems, characterized by high intensity and wide coverage, are prone to cause severe flooding and death in the middle and lower reaches of the Yangtze River valley (MLYR). This study aims to investigate the connection between PHREs and different time scales of atmospheric variability, thus elucidating the crucial factors and the influencing mechanisms for the prolonged PHREs during the mei-yu season. Our results reveal that the prolonged PHREs are dominated by the quasi-biweekly component of precipitation, while normal PHREs depend on the synoptic-scale component. The quasi-biweekly precipitation of prolonged PHREs is promoted and maintained by the meridional vertical circulation along the South China Sea (SCS)–MLYR, which is accompanied by the westward-propagating SCS QBWO-related suppressed convection at low latitudes. This study highlights the specific role of the low-latitude QBWO to the formation of prolonged PHREs relative to normal PHREs.